Rising CO2 enhances hypoxia tolerance in a marine fish

Global environmental change is increasing hypoxia in aquatic ecosystems. During hypoxic events, bacterial respiration causes an increase in carbon dioxide (CO2) while oxygen (O-2) declines. This is rarely accounted for when assessing hypoxia tolerances of aquatic organisms. We investigated the impact of environmentally realistic increases in CO2 on responses to hypoxia in European sea bass (Dicentrarchus labrax). We conducted a critical oxygen (O-2crit) test, a common measure of hypoxia tolerance, using two treatments in which O-2 levels were reduced with constant ambient CO2 levels (similar to 530 mu atm), or with reciprocal increases in CO2 (rising to similar to 2,500 mu atm). We also assessed blood acid-base chemistry and haemoglobin-O-2 binding affinity of sea bass in hypoxic conditions with ambient (similar to 650 mu atm) or raised CO2 (similar to 1770 mu atm) levels. Sea bass exhibited greater hypoxia tolerance (similar to 20% reduced O-2crit), associated with increased haemoglobin-O-2 affinity (similar to 32% fall in P-50) of red blood cells, when exposed to reciprocal changes in O-2 and CO2. This indicates that rising CO2 which accompanies environmental hypoxia facilitates increased O-2 uptake by the blood in low O-2 conditions, enhancing hypoxia tolerance. We recommend that when impacts of hypoxia on aquatic organisms are assessed, due consideration is given to associated environmental increases in CO2.